1. Field of the Invention
The present invention generally relates to exposure units, image forming apparatuses and image forming methods for forming an image by overlapping a plurality of color images using the electrophotography technique, and more particularly to an exposure unit, an image forming apparatus and an image forming method which form positional error correction patterns and correct positional errors of image forming positions of different colors by irradiating light on the positional error correction patterns and detecting the reflected light.
2. Description of the Related Art
A tandem type image forming apparatus has a plurality of image forming units for forming images of different colors, such as black, cyan, magenta and yellow images. A full color image is formed by overlapping color toner images formed by the image forming units.
In the tandem type image forming apparatus, the image forming positions of the image forming units, that is, the positions where the toner images of different colors are overlapped, deviate and do not match to cause the so-called color registration error. Consequently, it is impossible to obtain a stable full color image due to the color registration error. In order to correct the color registration error, the conventional image forming apparatus forms positional error correction patterns corresponding to the different colors, and detects the positions of the positional error correction patterns by a pattern detecting means such as an image sensor. The color registration error is corrected by controlling the overlapping positions of the positional error correction patterns corresponding to the different colors so that the overlapping positions match. As a result, the color registration error of the full color image caused by the positional errors of the image forming positions of the different colors is reduced in the image forming apparatus, to enable a more stable or high-quality full color image to be formed.
In another conventional image forming apparatus, the image forming units are configured to enable the more stable full color image to be formed. However, the structure of such image forming units is complex, and thereby increases the size of the image forming apparatus as a whole. A Japanese Laid-Open Patent Publication No. 2004-86088 proposes an image forming apparatus which can prevent such a size increase of the image forming apparatus. According to the proposed image forming apparatus, the exposure unit includes a plurality of scanner units each having a polygon mirror and a deflection mirror. Lights emitted from the plurality of scanner units irradiate a plurality of image bearing members. The plurality of scanner units are provided on the same vertical plane in order to accurately position the scanner units using a simple structure. This scanner unit arrangement stabilizes the quality of the full color image that is formed, and also reduces the mounting area of the scanner units within the image forming apparatus to thereby reduce the size of the image forming apparatus.
However, in the conventional image forming apparatuses, when a certain time elapses after correcting the positional errors of the image forming positions of the different colors, the positional errors are generated again due to various causes. For this reason, it is necessary to periodically perform the positional error correction.
Among other things, one cause of the positional errors that are generated when the certain time elapses after correcting the positional errors may be attributed to the positional errors of deflection mirrors that occur due to a temperature rise within the exposure unit.
The deflection mirror is fixed on a support member within the exposure unit using screws or an adhesive agent. But when the temperature within the exposure unit rises, the shape of the support member or parts used to secure the deflection mirror is deformed by the temperature rise within the exposure unit, and the inclination of the deflection mirror changes with respect to an optical path of the light irradiating the image bearing member.
When the temperature within the exposure unit rises, the amount of positional error increases within a relatively short time, and consequently, the positional errors need to be corrected at relatively frequent intervals. But while the positional errors are being corrected, the image forming apparatus cannot perform an image forming operation, and during this time, a user will regard this time as a down-time of the image forming apparatus. The presence of such a down-time deteriorates the performance of the image forming apparatus from the point of view of the user.
In order to reduce the down-time described above, it is necessary to prevent the amount of positional error from increasing with the temperature rise within the exposure unit, and to reduce the intervals at which the positional errors are corrected. However, no measures are taken in the conventional image forming apparatuses in order to prevent the amount of positional error from increasing with the temperature rise within the exposure unit, and to reduce the intervals at which the positional errors are corrected.